1. FIELD OF THE INVENTION
This invention relates to camshaft arrangements having a variable cam for opening preferably a load-change valve of an internal combustion engine and, more particularly, to a load-change valve camshaft arrangement providing for cushioned contact between the camshaft and the cam.
2. DESCRIPTION OF THE PRIOR ART
A camshaft arrangement having a variable cam is disclosed by German Offenlegungsschrift No. DE 32 34 640. The cam arrangement disclosed therein provides variable opening times for load-change valves of an internal combustion engine. In that arrangement, the camshaft has a portion projecting radially beyond the perimeter of the otherwise circular cross-section of the camshaft. The projecting portion is disposed within an internal recess of the cam element and is allowed restricted movement in the direction of rotation. Lateral bounding surfaces of the cam recess provide stops which restrict the angular motion of the radially projecting portion of the camshaft. The recess in the cam element communicates by way of a check valve with a supply passage in the camshaft for a hydraulic medium.
In the embodiment described in the German Offenlegungsschrift, the radially projecting portion of the camshaft has a throttling aperture so that the hydraulic medium will cushion relative motions between the cam element and the camshaft. The cross-section of the throttling aperture is independent of the rotational speed, hence the degree of cushioning varies with the rotation speed. This cushioning means is important, for example, when the posterior flank of the cam element is engaged by the valve stem after the point of maximum valve stroke has passed. At this time, the cam element is accelerated in the direction of the rotation of the camshaft by the restoring force of the valve. During this phase of operation, the cam element moves at a higher angular velocity than the camshaft, thus overtaking the latter. The impact resulting from this relative motion will generate unacceptable noise and cause damage without a cushioning means. In the prior art, this relative motion between the radial extension of the camshaft and the posterior stop in the cam element is cushioned by the aforementioned hydraulic medium, thereby avoiding such a hard impact.
However, because the cushioning means in the known arrangement employs a hydraulic medium within both the camshaft and the cam element, appropriate sealing measures are required. To avoid loss of hydraulic medium in the known arrangement, elaborate sealing precautions such as maintenance of close tolerances are necessary to provide for sealing of the recess in the cam element.